Quote:
Originally Posted by serialk11r
People with FI experience, how much headroom do the manufacturers usually leave in terms of knock resistance? I'm under the impression that moderate boost setups usually don't decrease compression ratio... Is it correct that for performance cars they usually design it to run on 91? Then with 93 a little boost wouldn't be much of a problem right?
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i couldn't say for sure what the manufacturing standard is for knock resistance in an engine. That sounds like a engineering question for someone in the industry, which is really for only a hand full of people in the world.
From my experience, cars that have been offered from the factory with forced induction had a few points of compression removed to make the engine perform more reliable. Even if stock boost pressures are a mear 5psi on small turbos.
Today in many more modern cars, knock is controlled via ECU or a separate device that detects pinging and tells the ECU to either reduce boost pressure or pull or add timing. So we can effectively run low-octane unleaded AND high-octane unleaded in performance cars without worrying about knocking.
So i guess to answer your question, if we turn up the boost moderately we have to make all other conditions equal during the 4 stroke process, so we have to turn up the fuel, and pull timing. This makes the compression ratio virtually unchanged allowing you to run the same amount of low-octane fuels safely and effectively, and 93 octane or similar high-octane ratings wont really have a major effect on the performance if not just better peace of mind.
When we start bumping up internal pressures from factory settings is when we need to start thinking about high octane to reduce risk of knock.
So not all performance cars have really high compression ratios from the factory, including ratios added from FI. That force makers to build the engine around certain octane ratings. But some engines have been tested and seen to run more optimally on higher octane ratings due to their nature. Probably because they have pretty high factory internal pressures or aggressive factory timing maps for performance.